The present invention relates to a multilayered sliding member having in the sliding surface thereof a lubricating composition mainly composed of polytetrafluoroethylene (hereinafter abbreviated as PTFE). Particularly, the present invention relates to a multilayered sliding member such as guide bush for shock absorber and bearing for hydraulic pump, which have in the sliding surface thereof a lubricating composition mainly composed of PTFE and exhibit excellent sliding characteristics in use in oil or under oil lubrication.
PTFE has been widely used as a starting material sliding members such as bearings for its peculiar characteristic properties such as excellent self-lubricating properties, low coefficient of friction, and high chemical and heat resistance. However, a sliding member made of PTFE alone is poor in wear resistance and creep resistance. To overcome this problem, several measures have been taken according to the purpose of use of the sliding members, such as (1) the blending of a filler such as graphite, molybdenum disulfide, glass fiber, etc., to PTFE, and (2) the impregnating and coating of a lubricating composition in and on a porous sintered metal layer deposited integrally on a thin steel plate.
A sliding member concerning the above-mentioned (2) is proposed in JP-B-39-16950. This JP-B-39-16950 discloses a bearing material comprising a matrix composed of a porous copper or copper alloy layer which may be disposed on a steel backing, and a mixture of polytetrafluoroethylene and lead and/or an oxide thereof, which the mixture is impregnated at least at the exposed face of the porous copper or copper alloy layer, the amount of this mixture being not less than 28% in volume based on the matrix, and the amount of lead or lead and an oxide thereof being at least 16% in volume based on the matrix.
However, the conventional sliding members such as disclosed in the above, although are low in coefficient of friction and admirable in frictional properties, were not necessarily satisfactory in wear resistance. Also, the conventional sliding members had the problem that in use in oil, particularly in a state of high-speed sliding, the sliding surface was prone to damage by cavitation caused in such sliding, promoting abrasion of the sliding member.
As a result of the present inventors' earnest studies for overcoming the above prior art problems, it has been found that a multilayered sliding member obtained by scattering and spreading a wet lubricating composition comprising a mixture of polytetrafluoroethylene and 1 to 40 wt % of (i) zinc oxide whiskers or (ii) zinc oxide whiskers and fibrous magnesium oxysulfate and/or fan-shaped magnesium oxysulfate as a filler and a petroleum solvent, on a porous sintered metal layer formed on a steel backing, rolling the resultant product by a roller so as to obtain a lubricating composition impregnated in and coated on the sintered metal layer, followed by heating to evaporate away the petroleum solvent in the lubricant composition, and then baking the resulting product by heating at a temperature of 360.degree. to 380.degree. C., shows a more low and stable coefficient of friction under oil lubrication, and also exhibits more excellent wear-resistant and anti-cavitation properties. The present invention has been attained on the basis of this finding.